Motor gasoline formulations are expected to change in order to meet ever restrictive governmental regulations and competition from alternative fuels, such as methanol. One requirement for future gasolines is that they be substantially reduced in benzene content.
While various techniques can be used to selectively remove benzene from gasoline boiling range streams, the use of solid adsorbents, such as molecular sieves, presents advantages over other techniques such as distillation and solvent extraction. Distillation is not suitable primarily because benzene, which has a normal boiling point of about 176.degree. F., forms low boiling azeotropes with normal hexane and naphthenes, such as methyl cyclopentane and cyclohexane. Efficient separation of the benzene from the paraffinic compounds by distillation is not possible because the azeotropes tend to come overhead with the paraffinic compounds. These azeotropes boil in the same range as does normal hexane in a light naphtha cut, i.e., 150.degree. to 160.degree. F. Once the benzene is removed, this separation is simplified. Extraction with a solvent, such as sulfolane, is technically feasible, but presents some disadvantages. These disadvantages include the use of special equipment to compensate for the corrosive nature of sulfolane, and the appearance of sulfur impurities in the gasoline product.
Solid adsorbents have been used in the past for removing all the aromatics from the non-aromatic fraction of a mixed hydrocarbon stream. For example, U.S. Pat. No. 2,716,144 teaches the use of silica gel for separating all aromatics from gasoline or kerosene fractions. The silica gel containing adsorbed aromatics can then be desorbed with a suitable desorbent, such as an aromatic containing hydrocarbon, having a boiling point different than the benzene-containing process stream which is passed over the adsorbent. Other U.S. patents which teach the use of silica gel for adsorbing aromatics from a process stream, followed by desorption by use of a liquid hydrocarbon include U.S. Pat. Nos. 2,728,800; 2,847,485; and 2,856,444.
The separation of aromatics from process streams by use of a molecular sieve is taught in U.S. Pat. No. 3,963,934. In that patent a 13X molecular sieve is taught to adsorb not only aromatics, but also olefins and sulfur from a C.sub.5 /C.sub.6 naphtha stream prior to isomerization. U.S. Pat. No. 3,992,469 also teaches the use of molecular sieves for separating all aromatics from process streams. Type X and type Y crystalline aluminosilicate zeolites are taught as preferred molecular sieves. Also, U.S. Pat. No. 4,014,949 discloses that partially hydrated NaY gives a separation factor of 1.6 for benzene (adsorbed) with toluene.
While much work has been done to separate aromatics from non-aromatics in process streams, there is still a need in the art for selectively removing benzene from both the other aromatic and the non-aromatic components of the stream. The need to remove benzene from gasoline boiling range streams will be more critical in the future because of more stringent government requirements.